The Mato Grosso branch of Aprosoja, the association representing the growers that filed the lawsuit at a federal court in Brasília on Wednesday, claimed "the patent does not fully reveal the invention so as to allow, at the end of the exclusivity period, any person can freely have access to it."

That requirement "avoids that a company controls a technology for an undetermined period of time," Aprosoja said, adding Intacta's patent protection extends through October 2022.

Monsanto did not have an immediate comment.

Mato Grosso farmers are leading a push in Brazil to replace genetically modified soybeans with non-GM seeds.

With about 53 percent of Brazil's soy area planted with Intacta technology in the 2016/17 crop cycle, Monsanto is a dominant force, Aprosoja says citing data from consultancy Agroconsult.

Some 40 percent of the country's area is grown with Monsanto's Roundup Ready seed technology and only 7 percent is non-GM, the data show.

This is the second time Mato Grosso farmers challenged Monsanto in Brazil, its most important market outside the United States. In 2012, Aprosoja claimed Monsanto was charging royalties over a patent that had expired two years prior.

By 2013, after the legal disputes, Monsanto had stopped collecting royalties linked to its first-generation Roundup Ready technology, Intacta's predecessor, according to Aprosoja. At that point, some farmers agreed to get a discount rate to use Monsanto's newer Intacta seeds during four years, it said.

Monsanto, which is being acquired by Bayer AG, is also facing close scrutiny from regulators concerning this deal.

As one condition to approve it in Brazil, Aprosoja's national branch is seeking to persuade local competition watchdog Cade to force the biotech company to sell its Intacta soy seed technology.

Biotech crops are genetically engineered to resist pests or disease, tolerate drought or withstand sprayings of weedkillers like glyphosate, the active ingredient in Monsanto's Roundup herbicide.

Europe has been stuck over what to do with the herbicide glyphosate that critics say could cause cancer.

In the United States, Monsanto is facing lawsuits over its dicamba herbicides, which is allegedly damaging crops unable to tolerate it in areas nearby where it was sprayed. (Reporting by Ana Mano; Editing by Marguerita Choy)